Somatostatin was isolated from ovine hypothalmic tissue, as a peptide (SST-14) composed of 14 amino acids and having a growth hormone secretion-inhibiting effect. At present, somatostatin (SST-28) composed of 28 amino acids has been isolated and identified. These somatostatins are brain and enteric canal peptides broadly distributed not only in hypothalami but also, for example, cerebra, limbic systems, spinal cords, vagal nerves, autonomic ganglia, mucous membranes of digestive tubes, pancreatic Langerhans' islets, etc., and they inhibit the secretion of pituitary and digestive canal hormones such as growth hormone, thyrotropin, gastrin, insulin, glucagon, etc. In addition, they also inhibit gastric acid secretion, pancreatic exocretion, and digestive canal movement and blood streams. At present, type I to type V somatostatin receptors (SSTR1, SSTR2, SSTR3, SSTR4, SSTR5) are known, and it is recognized that they express different functions in central and peripheral sites of the body.    [1. Life Sciences, Vol. 57, No. 13, p. 1249, 1995;    2. Journal of Clinical Endocrinology and Metabolism, Vol. 80, No. 6, pp. 1789–1793;    3. The New England Journal of Medicine, Jan. 25, 1996;    4. Eur. J. Clin. Pharmacol., 1996, 51, 139–144;    5. Exp. Opin. Ther. Patents (1998) 8 (7): 855–870.]
At present, peptidic somatostatin analogues that inhibit specific hormone secretion have been clinically developed.